1. Field of the Invention
The invention is directed to a method of producing styrene polymers having a narrow particle size distribution. More specifically, the invention is directed to a process of preparing narrow particle size distribution styrene-containing polymers, in which the particle size distribution is controlled by the addition of metal carbonates and/or bicarbonates to the polymerization mixture.
2. Discussion of the Background
Styrene polymers can be obtained by the procedure of so-called bead polymerization or suspension polymerization in an aqueous dispersion. This process is used especially in the production of expandable or foamable styrene polymers. Water-soluble organic polymers that are called protective colloids are ordinarily used as suspension stabilizers. Finely divided powders such as calcium or barium sulfate or calcium phosphate can also be used to stabilize the suspension droplet. Such stabilizer systems are called Pickering stabilizers. A list of industrially used protective colloids can be found, for example, in the article by Trommsdorf and Munster in Schildknecht: Polymer Processes Vol. 29, pages 119 to 120.
The choice of suitable protective colloids has substantial importance for the following reasons:
1. Adjusting for Narrow Particle Size Distributions with Definite Size
Foamable styrene polymers are used for various purposes depending on the bead size; coarse beads (2.5 to 0.8 mm) are used for the production of insulating panels, while finer fractions (0.8 to 0.4 mm in diameter) are used for the production of packing material. It is therefore necessary for beads in the particular desired particle size range to be formed in adequate amount, i.e., in high yield.
The occurrence of oversized or undersized grains should be as small as possible. Conventional processes do permit a certain control of the particle size itself, but the distribution curve is still always relatively broad, i.e., the proportion of so-called oversized and/or undersized grains is still too large.
2. Low Internal Water Content of the Beads
In conventional suspension polymerization, it is well known that a certain amount of water is always enclosed in the beads. Polymers with a low content of enclosed water have a uniform foam structure in the foamed state which has a positive effect on the thermal insulation capability of the foam panels. It is therefore desirable for the content of enclosed water, so-called internal water, to be as low as possible.
3. Spherical Shape of the Beads
In the suspension polymerization of styrene free of foaming agent, deformed beads are desirable because of their better processability. In the production of expandable styrene polymers, however, the beads should have as spherical in shape as possible.
4. Adequate Stability of the Suspension Throughout the Entire Polymerization Cycle
The suspension for producing expandable styrene polymers is even more labile than that of styrene polymers free of foaming agent. With the reactor sizes customary today of up to 100 m.sup.3, the loss of a batch represents substantial damage. It must therefore be guaranteed that in case of problems, the phase separation proceeds so slowly that sufficient time remains to add a polymerization inhibitor.
None of the suspension systems known up to now satisfy all of these requirements at the same time. There has, indeed, been no lack of attempts to find a practical way to meet all four requirements at the same time. As shown by the disclosed art, however, these efforts have been unsuccessful.
A process is described in DE-A-25 48 524 in which suspension stabilizers are used that have been produced by radical polymerization of styrene in the presence of polyvinylpyrrolidone.
DE-B2-25 10 937 describes a method in which the system is initially stabilized weakly in a state of low viscosity with tricalcium phosphate, and post-stabilization occurs several hours later with an aqueous solution of polyvinylpyrrolidone.
It is said that styrene polymers with low internal water content can be produced by both procedures. However, these procedures have the drawback that the particle size of the polymer is determined by the time of addition of the organic protective colloid.
The precise determination of the polymerization conversion in heterogeneous mixtures such as those present during suspension polymerization is difficult. Precise knowledge of the conversion is necessary, however, for reproducible adjustment of the particle size distribution, since the bead size depends on the particular viscosity of the polymerizing phase at which the protective colloid is added.
The polymerization system is also in an uncertain operating state for approximately two hours, which is a particular drawback when using large reactors. A problem such as stirrer failure can lead to destruction of the reactor, particularly at the beginning of polymerization when most of the styrene is still present.
It is proposed in DE-B-20 49 291 to use two protective colloids, namely polyvinyl alcohol with different degrees of hydroxylation, to obtain round beads of uniform size. As the examples indicated show, the ratio of styrene to water chosen must be so unfavorable that the process is uneconomical. The process does not contribute to the selective control of the particle size of the beads.
As already mentioned above, water-insoluble inorganic powders are also used as suspension stabilizers. The use of calcium phosphates is most common. These inorganic compounds as a rule are used in combination with smaller amounts of emulsifiers or surfactants (Houben-Weyl, Methoden der organischen Chemie, 4th Edition, Volume XIV, Part 1, Macromolecular substances, page 425). The use of these systems is restricted in comparison with organic protective colloids, however, since reproducible manipulation and progress of the suspension polymerization without problems is possible only in a narrow range. It is stated in this regard on page 422, last paragraph, lines 6 to 8 of this reference:
No conditions can be stated under which a powdered dispersing agent would be capable of broader application. With the combination of inorganic compounds with surfactants, the optimal dose must be complied with exactly, since both an underdose and an overdose of the surfactant can result in coagulation of the batch. PA1 a) an organic phase comprising at least 50 wt.% styrene and a monomer-soluble polymerization initiator, and PA1 b) an aqueous phase comprising water, an organic protective colloid and a substantially water insoluble inorganic suspension stabilizer;
It is also known from French Patent 20 79 991 how to modify the shape of the beads either by the amount of suspending agent (protective colloid) or by varying the phase ratio of aqueous to organic phase, or by using a mixture of organic protective colloid and inorganic suspension stabilizer. Spherical beads, or beads with low water content, are not necessarily obtained by this method, because the suspending agent is not added to the aqueous phase prior to polymerization. If the suspending agent is added at the beginning of polymerization, the particle size cannot be set reproducibly.
The similar process of U.S. Pat. No. 3,222,343 also does not meet the required conditions.
A process of suspension polymerization is disclosed in U.S. Pat. No. 3,222,340 which operates in the presence of calcium phosphate as suspension stabilizer. To improve the efficacy of the phosphate, relatively large amounts of a complexing agent are added. This reference does not teach how to narrow the particle size distribution of foamable polystyrene in suspension polymerization in the presence of organic protective colloids.